I think breaking the API/ABI because of required changes, is right,
breaking API/ABI because a fixed number of months has passed doesn't make
much sense.

From time to time, it is nice to get rid of deprecated APIs, to have
less code to maintain. GTK+ 3 has quite a lot of deprecated classes. So
getting rid of those, that alone is a good reason to release GTK+ 4, in
my opinion.

Tons of the deprecated APIs have no nice replacement, or no replacement
at all. For example GtkImageMenuItem, Stock Items, GtkActivatable.
Removing them will just make adoption of Gtk 4 even harder.